(Meth)acryloyloxyalkyl isocyanates typified by methacryloyloxyethyl isocyanate are industrially very useful compounds containing, per molecule, an isocyanate group having high reactivity with a compound having an active hydrogen, such as compounds having a substituent e.g. a hydroxyl group, or primary or secondary amino group, and a carbon-carbon double bond capable of vinyl polymerization. They are used in various purposes, for example, paints and coating materials, adhesives, photo-resists, dental materials and magnetic recording materials.
As described in U.S. Pat. No. 2,821,544 and JP-A-S54(1979)-5921, these compounds are prepared by using phosgene and generally contain impurities called as hydrolyzable chlorine.
When urethane acrylate and the like are prepared using a (meth)acryloyloxyalkyl isocyanate containing hydrolyzable chlorine, the hydrolyzable chlorine works as a catalyst poison and also a chlorine compound contaminated in a product affects the weathering resistance and corrosion resistance of the resulting product. Particularly, when the (meth)acryloyloxyalkyl isocyanate containing hydrolyzable chlorine is used to photo-resist materials for electronic apparatus parts, the presence of hydrolyzable chlorine may induce a serious problem.
Conventionally, various methods of decreasing the amount of hydrolyzable chlorine in an isocyanate compound are proposed.
There are known methods, for example, a method of mixing a hydrolyzable chlorine containing isocyanate compound with a minute alkali metal carbonate at a high temperature for a long period of time, a method of adding a zinc carboxylate and a hindered phenol oxidizing agent to a hydrolyzable chlorine containing isocyanate compound, subjecting to heat treatment, followed by distillation, a method of synthesizing an isocyanate in a solvent insoluble in water, and washing with a sodium hydrogen carbonate aqueous solution, and a method of treating a hydrolyzable chlorine containing polymethylene polyphenyl isocyanate with an epoxy compound. Furthermore, as a method without using a drug such as alkali metal carbonate and the like, there is a method of vaporizing a hydrolyzable chlorine containing isocyanate compound and condensing at a temperature of not lower than 70° C.
These methods, however, cannot decrease the amount of hydrolyzable chlorine sufficiently, or have various problems for solving in order to carry out them industrially. For example, in the method of mixing a hydrolyzable chlorine containing isocyanate compound with a minute alkali metal carbonate at a high temperature, it is difficult to separate the isocyanate compound and the carbonate after the treatment so that the occurrence of loss cannot be avoided. In the method of washing with a sodium hydrogen carbonate aqueous solution, white insoluble matters are deposited on the boundaries between an organic phase and a water phase and the matters cause obstacles in the subsequent separation procedure, or apparatus pollution. In the method of using the sodium salt, the isocyanate compound is liable to be contaminated by sodium ion, and even if the sodium ion content is in the level of ppm, it causes a serious problem in the case of using the isocyanate compound to electronic materials.
In particular, when an isocyanate compound having a carbon-carbon double bond is purified, it is desired to prevent the isocyanate compounds from polymerization reaction and simultaneously to decrease the content of hydrolyzable chlorine effectively. However, the above method cannot lead to the satisfactory results.
U.S. Pat. No. 4,310,688 discloses that a methylene chloride solution of methacryloyloxyethyl isocyanate containing 0.21% of hydrolyzable chlorine is treated with a compound containing a vicinal epoxy group (example: 1,2-butylene oxide) to decrease the hydrolyzable chlorine content to 0.05% by mass. However, this method can decrease the hydrolyzable chlorine content to several hundred ppm at the most, and the purified isocyanate compound prepared by this method is insufficient for the use including electronic materials and the like.
In order to solve the above problems associated with the prior arts, a method of treating with an epoxy compound in the presence of an amine is disclosed as a method of decreasing the hydrolyzable chlorine content in the isocyanate compound (JP-A-H9(1997)-323968. This method is an excellent method, but it cannot remove hydrolyzable chlorine completely. Therefore, the advent of a method of further decreasing the hydrolyzable chlorine content has been desired.
JP-A-H11(1999)-228523 discloses a method of preparing a (meth)acryloyloxyalkyl isocyanate substantially free from hydrolyzable chlorine by adding an amine and an epoxy group containing compound, heating and then purifying with distillation to prepare a purified acryoyloxyalkyl isocyanate substantially free from 2-chloro propionic acid isocyanate alkyl ester, or to prepare a purified methacryloyloxyalkyl isocyanate substantially free from 2-methyl-2-chloropropionic acid isocyanate alkyl ester. This method, however, has many problems in industrialization thereof such that in the case of carrying out distillation once after the addition of an amine and an epoxy group-containing compound, one having a hydrolyzable chlorine content of only 29 ppm is prepared and further, in order to further decrease the hydrolyzable chlorine content, it is necessary to conduct precision distillation.
Meanwhile, a method of preparing an isocyanate compound without using phosgene has been studied. With regard to (meth)acryloyloxyalkyl isocyanates, a method of pyrolysis of an urethane compound is proposed. The process comprises the step of pyrolysis at a high temperature, but the yield is not satisfactory economically because the (meth)acryloyloxyalkyl isocyanate is very easily polymerizable. Furthermore, a method of preparing an isocyanate compound at a relatively lower temperature by making an urethane compound into a dealkoxy silane with silanization is proposed, but in this method, an expensive compound is necessary and unnecessary wastes are produced. Further, this method has no description on the yield, so that it has many economic problems in industrialization.